1. Field of the Invention
This invention generally relates to musical tone generating apparatuses for use in electronic musical instruments, and more particularly to a musical tone generating apparatus in which a data compression method is used to reduce the amount of stored data from which tone signals are generated.
2. Prior Art
The electronic musical instruments suffer a big problem that it is hard to generate musical tones which are similar to those of acoustic musical instruments. Various kinds of musical tone signal generating methods are known as for the electronic musical instruments. Among these methods, a PCM (Pulse Code Modulation) method is superior to other methods because the PCM method can generate musical tones which are the most similar to those of the acoustic musical instruments. According to the PCM method, each of instantaneous values of the musical tone waveforms of the acoustic musical instrument are successively sampled and are stored in a memory as sampled digital data. The sampled digital data are read from the memory and are used for generating musical tones. Such PCM method is disclosed, for example, in U.S. Pat. No. 4,383,462 (the title of the invention thereof is "ELECTRONIC MUSICAL INSTRUMENT").
The musical tone waveforms generated by an acoustic musical instrument delicately depend on tone pitches, tone ranges and the like. A piano generates musical tone waveforms which differ by each tone pitch (or each tone range), for example. Of course, periods of such waveforms differ by each tone pitch. Hence, such waveforms must be memorized in the memory for each instrument and for each tone pitch (or each tone range), if necessary in the case where the musical tones truly similar to those of the acoustic musical instruments are to be generated based on the PCM method. For this reason, a scale of a memory storage becomes large. For reducing the amount of data, various kinds of methods, such as a DPCM (Differential Pulse Code Modulation) method, an ADPCM (Adaptive Differential Pulse Code Modulation) method, a DM (Delta Modulation) method, and an ADM (Adaptive Delta Modulation) method, have been proposed. And, the present applicant has disclosed a musical tone generating apparatus in Japanese Patent Application No.59-212382 for reducing the amount of data. In this apparatus, the data are compressed by use of the linear predictive coding and are written into the memory.
On the other hand, the present applicant has disclosed another musical tone generating apparatus in U.S. Pat. No. 4,520,708 for further reducing the amount of data. In this apparatus, the musical tone waveform is divided into an attack portion in which the envelope amplitude of such waveform abruptly increases and a sustain portion in which the envelope amplitude of such waveform is kept approximately constant. And the memory memorizes the attack portion and the selected part of the sustain portion (hereinafter, the selected part of the sustain portion will be referred to as a repeat portion). When the whole waveform must be generated, the attack portion is first read from the memory, and thereafter, the repeat portion is read from the memory repeatedly.
In the case where the compressed data are read from the memory and are decoded into the original data, the value of presently decoded data is obtained by use of the value of precedingly decoded data and the value of present data read from the memory. In this case, the data must be decoded so that the decoded data is continuous and smooth between the attack portion and the repeat portion and between the first repeat portion and the next repeat portion as well. However, there has been proposed no apparatus for realizing a decoding process by which continuous data is reproduced. Hence, the apparatus must memorize the whole waveform of each musical tone and the memory storage becomes large thereby.